You can sink a lot of time into reverse engineering Google's "Did you mean" reaction.
Here are a few random examples;
We can see that Google thinks 100 billion dvds are more likely than 100 billion videos.
However, Google also thinks that 100 billion smurfs is more likely than 100 billion eggs.
Google wonders whether you really mean 100 billion dollars... isn't it possible that you mean 100 million dollars? Whereas it has less reason to suggest an alternative if you search for 100 billion pounds.
Of course; I'm warping the logic here :)
A few strong indicator as to whether Google has an alternative suggestion to you is the number of matches it finds. A search for [I mean 100 billion pounds] brings up about a half million results. A search for [I mean 100 billion dollars] brings up eight million results.